This invention relates to well logging investigation of subsurface formations and, more particularly, to nuclear magnetic resonance (NMR) methods for determining characteristics of subsurface rock, including their pore characteristics.
General background of nuclear magnetic resonance (NMR) well logging is set forth, for example, in U.S. Pat. 5,023,551. Briefly, in NMR operation the spins of nuclei align themselves along an externally applied static magnetic field. This equilibrium situation can be disturbed by a pulse of an oscillating magnetic field (e.g. an RF pulse), which tips the spins away from the static field direction. After tipping, two things occur simultaneously. First, the spins precess around the static field at the Larmor frequency, given by xcfx890=xcex3B0, where B0 is the strength of the static field and xcex3 is the gyromagnetic ratio. Second, the spins return to the equilibrium direction according to a decay time T1, the spin lattice relaxation time. For hydrogen nuclei, xcex3/2xcfx80=4258 Hz/Gauss, so, for example, for a static field of 235 Gauss, the frequency of precession would be 1 MHz. Also associated with the spin of molecular nuclei is a second relaxation, T2, called the spin-spin relaxation time. At the end of a ninety degree tipping pulse, all the spins are pointed in a common direction perpendicular to the static field, and they all precess at the Larmor frequency. However, because of small inhomogeneities in the static field due to imperfect instrumentation or microscopic material heterogeneities, each nuclear spin precesses at a slightly different rate. T2 is a time constant of this xe2x80x9cdephasingxe2x80x9d.
A widely used technique for acquiring NMR data, both in the laboratory and in well logging, uses an RF pulse sequence known as the CPMG (Carr-Purcell-Meiboom-Gill) sequence. As is well known, after a wait time that precedes each pulse sequence, a ninety degree pulse causes the spins to start precessing. Then, a one hundred eighty degree pulse is applied to keep the spins in the measurement plane, but to cause the spins which are dephasing in the transverse plane to reverse direction and to refocus. By repeatedly reversing the spins using one hundred eighty degree pulses, a series of xe2x80x9cspin echoesxe2x80x9d appear, and the train of echoes is measured and processed.
The determination of the characteristics of the pores in subsurface rock formations is an important goal of NMR logging. Statistical description of the pore space is useful in understanding commercially important properties of the formations, such as permeability to fluid flow.
Existing NMR relaxation techniques typically determine T1 or T2 distributions which arise from surface relaxation. [For background, reference can be made, for example, to W. E. Kenyon, Nucl Geophys. 6, 153, 1992; R. L. Kleinberg, in xe2x80x9cEncyclopedia of Nuclear Magnetic Resonancexe2x80x9d, Wiley, N.Y., 1995; E. O. Stejskal and J. E. Tanner, J. Chem. Phys. 42,288, 1965; P. T. Callaghan, A. Coy, D. MacGowan, K. J. Packer and F. O. Zelaya 351, 467, 1991).] The data is generally analyzed in terms of distribution of (xcfx81S/V), where xcfx81 is the surface relaxivity and S/V is surface to volume ratio. The resulting pore size distribution is used, in turn, to obtain permeability, FFI (free fluid index), etc. While generally successful, this method suffers from xcfx81 being unknown and subject to wettability conditions. A few monolayers of an absorbent on the grains can change the surface relaxivity xcfx81 completely. There is also uncertainty regarding the proper cutoff to be used in estimating FFI.
It is among the objects of the present invention to provide an NMR well logging technique that can determine pore characteristics of formations, independent of wettability.
A form of the present invention includes a technique for discriminating and analyzing components of fluids using internal field gradient as a measure of pore size in rocks, substantially independent of wettability. When a porous material is subjected to a uniform external magnetic field, an inhomogeneous magnetic field may appear inside the pore space, due to the contrast of the magnetic susceptibility between the solid materials and the pore-filling fluid. The inhomogeneity of this internal field can be rather large in sedimentary rocks (see M. D. Hurlimann, J. Magn. Res. 131, 232-40, 1998).
As noted above, wettability can have a profound effect on surface reflectivity, resulting in uncertainty in determination of formation characteristics including pore size distribution, permeability, and free fluid index. In contrast, the strength of internal gradients are little affected by wettability. Therefore, NMR methods based on surface relaxation and internal gradients react differently to wettability changes.
A form of the present invention uses the decay due to the internal gradients g as a probe of the pore size. In a simple model, the strength of the internal gradient g is proportional to the applied field B0 times the susceptibility difference xcex94"khgr" between grain and fluid divided by the pore-size lp, i.e.   g  ∝            ΔχB      o              I      p      
Thus, g has pore size information. xcex94"khgr" may be a constant to be determined from cuttings or correlations. At short times, the decay of transverse magnetization in the presence of g is well known. This gives a direct correlation between the decay in the presence of the gradient and the poresize.
The total NMR decay is influenced both by surface relaxation and diffusion in internal gradients. By partitioning the time of evolution into two sectors and varying them systematically, the two processes can be separated.
In an accordance with an embodiment of the invention, there is set forth a method for determining a characteristic of formations surrounding an earth borehole, comprising the following steps: (a) providing a logging device that is moveable through the borehole; (b) producing, from the logging device, a static magnetic field in the formations; (c) transmitting, into the formations, from the logging device, a first magnetic pulse sequence and receiving, during the first pulse sequence, magnetic resonance spin echo signals; the first pulse sequence having a portion during which spin echoes are subject to decay due to local magnetic field gradients in the formations, and having another portion during which spin echoes are not substantially subject to decay due to the local magnetic field gradients in the formations; (d) transmitting, into the formations, from the logging device, a second magnetic pulse sequence and receiving, during the second pulse sequence, magnetic resonance spin echo signals; the second pulse sequence having a portion during which spin echoes are subject to decay due to the local magnetic field gradients in the formations, and having another portion during which spin echoes are not substantially subject to decay due to the local magnetic field gradients in the formations; one of the portions of the second magnetic pulse sequence having at least one different pulse parameter than the corresponding one of the portions of the first magnetic pulse sequence; and (e) determining said characteristic of the formations from the spin echo signals of the first pulse sequence and the spin echo signals of the second pulse sequence.
In one embodiment of the invention, the different pulse parameter comprises respectively different pulse spacings, and in another embodiment of the invention the different pulse parameter comprises different durations of the respective another portions of the first and second magnetic pulse sequences.
In an embodiment of the invention, said another portion of the respective first and second magnetic pulse sequences comprise successive CPMGs, the CPMG pulse spacing of said portion being much longer than the CPMG pulse spacing of said another portion of the respective first and second magnetic pulse sequences. In this embodiment, the step (e) includes deriving a first T2 distribution from the spin echo signals of said first magnetic pulse sequence and deriving a second T2 distribution from the spin echo signals of said second magnetic pulse sequence, and further includes deriving, from the first and second T2 distributions, at particular values of T2, the product g2D where g is magnetic field gradient and D is the diffusion constant of the formation fluid. Also in this embodiment, the step (c) can further includes deriving a formation pore size distribution from values of g2D, and also deriving formation permeability.
Further features and advantages of the invention will become more readily apparent from the following detailed description when taken in conjunction with the accompanying drawings.